Crystalline molecular sieves occur naturally or are synthesized as fine crystalline bodies which for general utility in commercial adsorptive or catalytic processes are usually formed into agglomerates, preferably possessing as high a degree of attrition resistance and crush strength as possible without unduly affecting the adsorptive properties of the sieves. One method of agglomerating these finely crystalline materials is by combining them with a clay binder as described in U.S. Pat. No. 2,973,327, issued Feb. 28, 1961 in the name of W. J. Mitchell, et al. Whereas this prior technique provides a suitable agglomerate for a wide variety of industrial applications, it has been found that certain applications having a very low tolerance for attrition-produced particles or dust require a more strongly bound molecular sieve agglomerate.
Moreover, in certain other processes, e.g. refrigerant drying, environments exist which are chemically incompatible with some molecular sieve agglomerates. Refrigerants often contain halogenated hydrocarbons which can decompose and the decomposition products, e.g., hydrogen fluoride and hydrogen chloride, can react with both the internal active sites in the molecular sieve as well as the binder.
As a result of the above-described problems, processes have been developed to improve both the physical properties, i.e., crush strength and attrition resistance, and chemical properties, i.e., compatibility with halogenated refrigerants, of molecular sieve agglomerates.
U.S. Pat. No. 3,536,521 discloses a silicone coated molecular sieve and method for preparing same wherein a silicone coating is applied to the surface of a molecular sieve by dissolving a silicone oil in an appropriate solvent, adding the sieves to the resulting solution and evaporating the solvent, leaving a uniform silicone deposit on the sieve surface. The sieve is then thermally activated for use as a drying agent. The above-identified patent discloses that certain gases, e.g., refrigerants, are not adsorbed on the coated surface. However, no disclosure is provided relating to attrition and other physical breakage problems, nor to subsequent chemical attack of the new surfaces after such breakage has occurred.
Other methods have been proposed to incorporate silicon on the surfaces throughout the molecular sieve agglomerates to provide improved physical strength as well as chemical resistance. These methods commonly involve incorporating a silicate treatment step, i.e., contacting with an aqueous alkali metal silicate solution, into the molecular sieve agglomerate manufacturing procedure.
For example, U.S. Pat. No. 3,624,003 discloses molecular sieve agglomerates and methods for preparing same wherein improved, attrition resistant desiccant bodies are prepared by the process which comprises applying to the outer surface of a crystalline zeolitic molecular sieve agglomerate an essentially continuous coating of a finely divided inert alpha-alumina monohydrate which has been thermally treated at temperatures of from about 250.degree. C. to 350.degree. C. to reduce the surface activity thereof, contacting and impregnating at least the coating of the agglomerate thus formed with an aqueous solution of potassium silicate, drying the potassium silicate impregnated agglomerate to remove a substantial portion of water therein, and thereafter firing the resulting composite agglomerate to set and harden the silicate and activate the molecular sieve. This firing, or heating step, is sometimes referred to as calcination.
Similarly, U.S. Pat. No. 3,625,886 discloses molecular sieve agglomerates and methods for preparing same, however this patent discloses the use of a mixture of disapore, i.e., beta-alumina monohydrate, and a clay mineral, instead of the alpha-alumina monohydrate disclosed in U.S. Pat. No. 3,624,003.
Both of the above-identified patents disclose that the silicate treatment step is performed on the agglomerates prior to the heating step which sets, or hardens, the binder. That is, the silicate treatment is an integral step in the molecular sieve agglomerate manufacturing procedure.
Other patents also disclose this process sequence. British Patent Specification 972,833 discloses a method for hardening a crystalline zeolite molecular sieve agglomerate formed of such zeolite molecular sieve and a clay mineral binder, which comprises contacting the agglomerate in a hydrated state with an aqueous solution of an alkali metal silicate having a solid content of from 3% to 35% by weight to impregnate the agglomerate with the alkali metal silicate, separating the impregnated agglomerate from the solution and firing such impregnated agglomerate at a temperature of at least 343.degree. C. and below the temperature at which the crystalline zeolitic molecular sieve loses its structural ability.
Also, U.S. Pat. No. 4,405,503 discloses a method for strengthening zeolitic molecular sieve agglomerates, particularly of the bound variety, which enables them to retain (a) their strength to a satisfactory extent despite subsequent acid treatment and (b) their suitability for use as a catalyst support. The method comprises treating the agglomerate with an aqueous solution of water soluble silicon compound and subsequently with an aqueous solution of a mineral acid of sufficient strength to decationize the zeolitic molecular sieve and/or increase its SiO.sub.2 :Al.sub.2 O.sub.3 ratio. The decationized zeolitic molecular sieve is thereafter calcined.
Although the above-described methods for treating molecular sieve agglomerates have been useful, new methods are sought which could be performed subsequently to the binder setting step, e.g., calcination step. Such a method could be utilized in existing manufacturing facilities where intermediate method steps may not be conveniently implemented. Moreover, such a processing sequence could result in a high degree of product consistency, i.e., little variation in physical and chemical properties.